This invention relates to a plastic film having high moistureproofness. More specifically, this invention relates to a film of high moistureproofness which is transparent, substantially impermeable to gases such as water vapor, and well-suited for packaging films for EL, elements that are used for back lights in liquid crystal displays.
Liquid crystal displays have been steadily used as display devices for digital watches, electronic desktop calculators and so on, making much use of their low power consumption and in great demand with the advent of liquid crystal games. Ever since, they have found more diverse applications in motor cars, musical instruments, office and factory OA/FA equipment, etc. On the other hand, an organic dispersion type of electroluminescence (EL) elements are being now increasingly used as inexpensive planar-luminous back lights .(auxiliary light sources) for liquid crystal displays, taking the advantage of their thinness and lightness. The organic dispersion type of EL elements are packaged with transparent films with excellent moistureproofness, since the luminance of fluorescent substances such as ZnS:Mn and ZnS:Cu considerably decreases by moisture absorption.
Packaging films so far used for the organic dispersion type of EL elements employed for back lights in liquid crystal displays typically include laminated films composed mainly of fluorinated resin films, esp., polychlorotrifluoroethylene (PCTFE) thanks to their high moistureproofness and transparency.
As packaging films for the EL elements, use has been made of PCTFE films generally having a thickness of about 70 to 300 .mu.m and laminated thereon with a heat-sealing polyolefin sealant generally having a thickness of about 20 to 100 .mu.m. Among existing transparent plastic films, such laminated films have been said to possess the highest moistureproofness.
In recent years, on the other hand, transparent plastic films having on their surfaces a transparent thin film, which is formed of an oxide of a metal, inter alia, silicon or aluminium, have been commercially available as packaging materials having gas barrier properties.
However, the conventional laminated films providing packaging films for EL elements and composed primarily of fluorinated resin films, esp., polychlorotrifluoroethylene (PCTFE) are too expensive, thus leading to an increased production cost for back lights. Another problem with the laminated films composed mainly of PCTFE is that their moistureproofness degrades so considerably at an ambient temperature exceeding 50.degree. C. that the service life of EL elements is much reduced at such elevated temperatures.
Accordingly, there has been a strong desire in the art to obtain transparent plastic films which are less expensive and more moisture-resistant than PCTFE films and which are advantageously used as packaging films for the organic dispersion type of EL elements.
Moreover, the conventional transparent plastic films having a transparent thin film of a metal oxide formed on their surfaces are still insufficient in moistureproofness to use as packaging films for the EL elements used for back lights in liquid crystal displays.
This invention has been accomplished in the above background. Accordingly, it is the object of this invention to provide a material which .has an improved transparency and moistureproofness, is satisfactory in view of strength and economical considerations, and is particularly well-suited for packaging films for EL elements for back lights in liquid crystal displays, said packaging films being required to possess high moistureproofness.